FRAMs (Ferroelectric Random Access Memories) are becoming more and more popular. A FRAM is a non-volatile memory. A memory cell of a FRAM includes a ferroelectric capacitor having a ferroelectric material instead of a dielectric. A typical known ferroelectric used in FRAMs is PZT (lead zirconate titanate). This material is ferroelectric having a spontaneous electric polarization which can be reversed in the presence of an electric field. A reversed electric polarization remains reversed after the electric field is removed.
The ferroelectric material forms a crystal with a mobile atom that can move in the direction of an applied field. The polarity of the crystal depends on the position of this movable atom. The polarity remains once the electric field is removed. This preserves data within the memory without needing periodic refresh. Because there is no physical barrier, the atom may switch in less than 1 ns. FRAM writes are thus extremely quick. The write speed for FRAM is around 55 ns as compared to the much slower 5 ms write speed for EEPROMs. Thus FRAMs may be written to during one clock cycle. FRAMs also require much less write power than EEPROM and flash memories.
A read-access to a ferroelectric random access memory is a write-access followed by sensing. A “0” is written to discover the original data stored. If the original data was a “1”, writing a “0” would change the polarity in the cell. The central atom then passes through the crystal's center and emits a charge spike which can be sensed as a current spike on the sensing wire. If the original data was a “0”, there will be no charge spike. Thus sensing the presence of a charge or of current spike on the sensing wire determines the original data of the accessed cell.
FRAM memory technology has a drawback because each read-access thus destroys the content of the cells read contrary to other memory technologies. Therefore, after each read-access the data needs to be written back into the cells to restore the original content. The write-back is generally performed by a sense amplifier which is activated to drive the bit line to the full supply voltage if the voltage developed on the bit line corresponds to a stored data “1” and drive the bit line to 0 V if the voltage on the bit line corresponds to a stored data “0”.
The read-access time for a FRAM memory is about twice of the read-access time for an EEPROM or a flash memory because of this necessary write-back process. Thus EEPROMs and flash memories remain popular in applications that demand numerous memory reads but only occasional memory writes. On the other hand, ferroelectric memories are superior to EEPROMs and flash memories in the overall power consumption. Therefore together with the extremely fast write-access times, it would be an advantage to use FRAM memories in all kind of applications. However to use a FRAM as program memory, the write-back after a read-access must be ensured under all conditions. These conditions include shorts on the supply pins. Otherwise the integrity of the memory would not be guaranteed. If a FRAM memory includes program data and a short on the supply voltage occurs with a very fast voltage rupture, parts of the FRAM memory may be destroyed. This could lead to complete failure of the electronic device using the FRAM memory. In contrast, a short on the supply voltage when flash memory includes program data will only lead to a reset.
It is known in the state of the art to provide a large external capacitor in the range of μFs for stabilizing the supply voltage and to ensure a slow voltage drop. However such a capacitor is liable to accidental shorts.
There is a need for an electronic device using a FRAM memory which ensures that after a read-access the necessary write-back is performed even for the case of a short-circuit.
There is further a need for a method for granting read-access to a FRAM memory which ensures an undisturbed write-back process.